webdav returns "503 Service Unavailable" in case the ownCloud server is ...

...currently in maintenance mode.

@danimo @dragotin can you please make sure that mirall/csync handle 503 properly?
@evert PR about class Sabre_DAV_Exception_ServiceUnavailable will arrive soon. THX

Anybody else in review mood?
@karlitschek @butonic @bartv2 @blizzz @tanghus @owncloud/core-developers